1. Field of the Invention
The present invention relates to a method and apparatus for producing a silicon single crystal wherein a silicon single crystal ingot is grown by the Czochralski method in which a horizontal magnetic field is applied (Horizontal Magnetic-field-applied Czochralski Method, HMCZ method) and so forth.
2. Description of the Related Art
As a method for producing a silicon single crystal used for the production of semiconductors, the Czochralski method (CZ method) is widely used, in which a crystal is pulled from silicon melt contained in a quartz crucible while growing the crystal. In the CZ method, since heating is performed from the side of a crucible, natural convection occurs in the melt. Further, since rotation number of the crystal and rotation number of the crucible are controlled in order to obtain a silicon single crystal of high quality, forced convection is also generated in the silicon melt and a complicated flow is formed. It is said that a method of applying a static magnetic field to the silicon melt is effective in control of the convection in such a melt (refer to xe2x80x9cMagneticfield-applied CZ Silicon Crystal Growth and Characteristics thereofxe2x80x9d, Integrated Circuit Symposium, 1980. 11).
Such a method is widely known as a horizontal magnetic field type HMCZ method, and the production has been performed by using a lengthwise (vertical) magnetic field component at the surface of the melt with a magnitude of 0 or in an extremely small ratio (about 0.025) with respect to the lateral (horizontal) magnetic field component. This is because, in the HMCZ method, it is an important object to suppress the convection of the melt along the vertical direction to facilitate the growth of a single crystal.
By the way, in-the production of recent highly integrated semiconductor devices, interstitial oxygen atoms introduced into silicon wafers that serve as a substrate are utilized in various ways. For example, they are used for the improvement in mechanical strength for bearing the thermal stress in the device fabrication process, and they are used as gettering sites for heavy metal impurities as fine defects (Bulk Micro Defects) formed by precipitation of the interstitial oxygen atoms excessively introduced in the device fabrication process. Therefore, in recent high quality silicon single crystals, control and uniformity of the interstitial oxygen concentration are important.
However, in such a pulling method as mentioned above based on the HMCZ method, the convection of the silicon melt in a quartz crucible is suppressed, and therefore the crystal production is easy. However, minute variation in the interstitial oxygen concentration as crystal quality may be generated to degrade the production yield of single crystals. That is, there is generated variation in the interstitial oxygen concentration along the crystal growth axis direction with a length of about several hundreds microns to about several millimeters and an amplitude of about 1 ppma (JEIDA), and silicon wafers made from this portion would have markedly degraded oxygen concentration distribution for the planar direction of the wafers. Since this portion produces failure products, it reduces the yield, degrades the productivity and increases the cost in the production of silicon single crystals.
As for the magnetic field distribution in the HMCZ method, for example, Japanese Patent Laid-open Publication (Kokai) No. 62-256788 discloses that the magnetic field distribution is controlled so that it should conform to the curvature of the bottom or wall of a quartz crucible along the circumferential direction of the crucible. However, it discloses only the effect of prolonging the lifetime of heaters to be used, and the quality of the produced single crystals cannot be improved. Further, Japanese Patent Laid-open Publication No. 9-188590 discloses a method for improving quality of silicon single crystals based on the HMCZ method. However, this method cannot necessarily provide sufficient effect.
Therefore, the present invention was accomplished in view of such conventional problems, and its major object is to provide a method and an apparatus for producing a silicon single crystal, which can grow a silicon single crystal ingot having highly uniform interstitial oxygen concentration along the growth axis direction of the grown single crystal with high productivity and high yield by the CZ method wherein a horizontal magnetic field is applied.
According to the first aspect of the present invention for achieving the aforementioned object, there is provided a method for producing a silicon single crystal by growing a single crystal ingot while a magnetic field perpendicular to a crystal growth axis is applied to a silicon melt contained in a quartz crucible during pulling of the single crystal from the melt contained in the quartz crucible, wherein the crystal growth is performed so that one of a low temperature region and a high temperature region generated at a surface of the silicon melt contained in the crucible should always cover a solid-liquid interface of the crystal growth.
If the crystal growth is performed so that one of a low temperature region and a high temperature region generated at a surface of the silicon melt should always cover a solid-liquid interface of the crystal growth as described above, variation in the oxygen concentration along the growth direction generated during the crystal growth can be suppressed, and uniformity of the interstitial oxygen concentration within a plane along the radial direction of the crystal can be improved.
In the above method, the crystal growth can be performed so that one of the low temperature region and the high temperature region should always locate at the center of the surface of the silicon melt.
By this characteristic, the crystal growth becomes easy. In addition, the low temperature region or the high temperature region can cover a solid-liquid interface, and this state can be maintained stably for a long period of time. Therefore, the variation in the interstitial oxygen concentration along the direction of the crystal growth can further be suppressed, and improvements of productivity and yield of single crystals with highly uniform interstitial oxygen concentration can be realized.
Further, in the above method, the low temperature region or the high temperature region of the melt surface can be detected by a radiation pyrometer, thermocouple or CCD camera.
If the temperature distribution of the melt surface is measured with a radiation pyrometer, thermocouple or CCD camera to detect and confirm the position and size of the high temperature region or the low temperature region as described above, the high temperature region or the low temperature region can be easily detected and always located at the center of the melt surface. This is effective for suppressing the variation in the temperature distribution, and thus uniformity of the interstitial oxygen concentration along the single crystal growth direction can be improved.
Further, in the aforementioned method for producing a silicon single crystal, it is desirable that the monitoring of the temperature distribution of the melt surface with the aforementioned radiation pyrometer, thermocouple or CCD camera should be always performed continuously during the crystal growth, and thereby the crystal growth is performed so that one of the low temperature region and the high temperature region generated at the melt surface should always cover the solid-liquid interface of the crystal growth.
Further, the present invention provides the aforementioned method for producing a silicon single crystal, wherein crystal growth experiments can be preliminarily performed to obtain such conditions that one of the low temperature region and the high temperature region generated at the melt surface should always cover the solid-liquid interface of the crystal growth, and the conditions can be applied to the crystal growth operation based on the monitoring of the temperature distribution of the melt surface with the aforementioned radiation pyrometer, thermocouple or CCD camera.
The conditions that enable one of the low temperature region and the high temperature region generated at the melt surface to always cover the solid-liquid interface of the crystal growth consist of factors including the temperature distribution in the melt, direction, position and velocity of the convection of the melt, rotation speed of the growing crystal, rotation speed of the crucible, temperature distribution in the furnace, amount, flow rate and nozzle position of atmospheric gas in the furnace, intensity and magnetic field center position of the horizontal magnetic field, characteristics of the furnace and so forth, and these factors influence on one another in a complicated manner. Therefore, the factors must be narrowed down by preliminarily performing crystal growth experiments. Then, in an actual operation, the selected factors can be controlled by feeding back the detection results obtained by the radiation pyrometer, thermocouple or CCD camera to stabilize the temperature distribution of the melt surface, and this state can be maintained during the crystal growth to suppress the variation in the interstitial oxygen concentration in the growing crystal.
The present invention further provides a silicon single crystal produced by the aforementioned method, which is a silicon single crystal that has an-extremely high uniformity of the interstitial oxygen concentration in the crystal along the growth direction.
The present invention still further provides a silicon single crystal wafer obtained from a silicon single crystal produced by the aforementioned method, which is a silicon single crystal wafer having an extremely reduced minute variation in the interstitial oxygen concentration distribution along the radial direction in a plane.
Further, the present invention also provides a silicon single crystal produced by the horizontal magnetic field type Czochralski method, wherein variation magnitude of the interstitial oxygen concentration is 0.5 ppma or less in an arbitrary section with a length of 40 mm along the crystal growth axis direction of the single crystal pulled from a silicon melt contained in a quartz crucible. This crystal is a silicon single crystal having an extremely reduced minute variation in the interstitial oxygen concentration distribution along the crystal growth axis direction.
Further, as the second aspect of the present invention for achieving the aforementioned object, there is provided a method for producing a silicon single crystal by growing a single crystal ingot while a magnetic field perpendicular to a crystal growth axis is applied to a silicon melt contained in a quartz crucible during pulling of the single crystal from the melt contained in a quartz crucible, wherein a ratio of vertical magnetic field component to horizontal magnetic field component for magnetic field strength at the crystal center of the surface of the silicon melt contained in the quartz crucible is 0.3 or more and 0.5 or less.
If the ratio of the vertical magnetic field component to the horizontal magnetic field component for the magnetic field strength at the crystal center of the surface of the silicon melt contained in a quartz crucible is 0.3 or more and 0.5 or less as described above, the vertical magnetic field component is increased and thus the lines of magnetic force are curved as for the vertical direction. Therefore, the whole convection of the melt can be suppressed, and the convection horizontally flowing at the melt surface is suppressed at the same time. This makes it possible to suppress the variation in the interstitial oxygen concentration along the direction of the crystal growth axis, and productivity and yield of high quality silicon single crystal can be improved.
In the aforementioned method, a position of a magnetic field generation apparatus and/or a position of the quartz crucible can be adjusted so that the ratio of the vertical magnetic field component to the horizontal magnetic field component for the magnetic field strength at the silicon melt surface during the single crystal growth should be within the range defined above.
By the above characteristic, it becomes possible to stably maintain the aforementioned magnetic field strength ratio at a desired level within the range of 0.3 or more and 0.5 or less for a long period of time, and thus it becomes possible to suppress the convection horizontally flowing at the melt surface. Therefore, the variation in the interstitial oxygen concentration along the crystal growth axis direction can be suppressed.
Further, in this case, a magnetic field strength adjusting member may be provided around a magnetic field generation apparatus or a single crystal growing apparatus, and the position thereof may be adjusted so that the magnetic field strength ratio at the silicon melt surface during the single crystal growth should be within the aforementioned range.
For example, even when the ratio of the vertical magnetic field component to the horizontal magnetic field component for the magnetic field strength is out of the range defined above because the positions of the magnetic field generation apparatus and the single crystal growing apparatus (quartz crucible) are limited by each other as relative positional relationship, the magnetic field strength ratio at the melt surface can be controlled within the range defined above by providing the magnetic field strength adjusting member around the magnetic field generation apparatus or the single crystal growing apparatus and adjusting the position thereof as described above. Thus, uniformity of the interstitial oxygen concentration of a growing crystal can be improved.
The present invention also provides an apparatus for producing a silicon single crystal by pulling a single crystal from silicon melt contained in a quartz crucible, which is provided with a magnetic field generation apparatus for applying a magnetic field perpendicular to the crystal growth axis to the melt in the quartz crucible, wherein a member for controlling a magnetic field strength ratio is provided around the magnetic field generation apparatus or the apparatus for growing a single crystal.
In an apparatus for producing a silicon single crystal provided with a magnetic field generation apparatus for applying a horizontal magnetic field, if a member for controlling a magnetic field strength ratio is provided around the magnetic field generation apparatus or the single crystal growing apparatus as described above, it becomes possible to obtain the ratio at the melt surface of the vertical magnetic field component/the horizontal magnetic field component for the magnetic field strength at a desired level. Thus, the apparatus can be an apparatus that can control the convection horizontally flowing at the melt surface to suppress the fluctuation in the interstitial oxygen concentration along the crystal growth axis direction, produce silicon single crystal of high quality with good yield, and improve the productivity and cost at the same time.
Because the-method and apparatus of the present invention for producing a silicon single crystal by the horizontal magnetic field applying Czochralski method can suppress the fluctuation in the interstitial oxygen concentration along the growth axis direction in a growing crystal and markedly reduce the minute variation in the interstitial oxygen concentration in a plane of wafer, they can industrially produce silicon single crystals of high quality with high yield at low cost.